We examined the oxygen isotopic systematics for ammonia oxidation, the first step in the regeneration of nitrate from ammonium. In particular, oxygen isotopic fractionation and exchange with water were evaluated for their roles in determining the delta O-18 of nitrite produced by four species of ammonia-oxidizing bacteria (AOB). Microbially catalyzed oxygen isotopic exchange between nitrite and water was less than 25% at low cell densities (106 cells mL(-1)) and ammonium concentrations (less than 50 mu mol L-1). The amount of exchange was relatively constant for a given species of ammonia oxidizer but varied between 1% and 25% among the four species tested. The delta O-18 value of nitrite produced at a given water delta O-18 value also varied by nearly 10% among the different species. Isotopic fractionation, either during oxygen (O-2) incorporation by ammonia monooxygenase and/or water incorporation by hydroxylamine oxidoreductase plays an important role in setting the delta O-18 of nitrite produced by AOB. This work provides a detailed characterization of the oxygen isotopic systematics of ammonia oxidation by AOB, which will help us better interpret the oxygen isotopic distributions of nitrite, nitrate, and nitrous oxide in terrestrial and aquatic environments.
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